This invention relates to a novel method for electrically processing completely-assembled cathode-ray tubes having electron guns, such as bipotential-focus guns, in which the focus electrode G3 is normally operated at voltages which are independent of the voltages on the control electrode G1, the screen electrode G2, and the high-voltage electrode G4. One or more guns may be installed in such cathode-ray tubes.
In the manufacture of cathode-ray tubes, it is the practice to process the tubes after they have been completely assembled so that the tube becomes operative, the tube operation is stabilized and the operating life is lengthened. For this processing, each gun in the tube is usually subjected in succession to the steps of "spot knocking," "hot shot," "high-voltage aging" and "low-voltage aging."
In one form of the "spot-knocking" step, the cathode, the heater and the low-voltage electrodes G1, G2 and G3 are grounded, and a pulsed positive voltage which peaks at about 200% of the normal ultor voltage is applied to the high-voltage electrode G4 and to the anode (the internal conductive funnel coating) of the tube for about 2 minutes to burn off loose particles which may reside between the electrodes in the gun.
In one form of the hot-shot step, the cathode is activated by heating it to an abnormally high temperature, as by applying about 10 to 12 volts across the cathode heater, where 6 to 7 volts are normally applied, for about 2 minutes, with all of the electrodes and the anode floating electrically.
In one form of the high-voltage aging step, which usually lasts for about 3 to 60 minutes, the cathode is emitting, various combinations of constant voltages including ground potential are applied to the G1, G2 and G3 electrodes, and a high voltage, substantially higher than normal operating ultor voltage, is applied to the high-voltage electrode G4. The high-voltage aging step allows time-related defects to manifest themselves and, in most cases, cure themselves.
In one form of the low-voltage aging step, sometimes called the cathode-aging step, which usually lasts for about 30 to 90 minutes, the cathode is emitting, various combinations of constant positive voltages are applied to the control electrode G1, the screen electrode G2 and the focus electrode G3, and the high-voltage electrode G4 is floating electrically. The low-voltage aging step permits the emission from the cathode to stabilize and the various electrodes to outgas due to bombardment by electrons from the cathode. Outgassing of the electrodes occurs principally during exhaust baking, particularly during rf induction heating of the electrodes just prior to tipping-off the tube, and again in each of the above-described processing steps. Nevertheless, significant amounts of gas remain in the screen electrode G2 and in the focus electrode G3 which outgas during the subsequent testing and/or during the operation of the tube. The effect of such subsequent outgassing is to lower the initial cathode-emission level from the cathode and/or to cause the cathode emission level to drop subsequently from its initial value.